This invention relates to field emission cathodes, and in particular to large-area field emission cathodes of semiconductor-metal eutectic composite materials.
High current density electron sources are required for a variety of laser and microwave tube applications. Development of the x-ray laser, in particular, requires high current density sources. At the present time this requirement is met by using thermionic cathodes, because of the high current densities possible with these devices. Unfortunately, thermionic cathodes are wasteful of power and cannot be turned on and off rapidly.
Field emission cathodes do not suffer from these drawbacks. Thus these "cold" cathodes would be attractive as possible alternatives to thermionic cathodes if reliable field emission cathode devices capable of yielding high current densities and high gross currents could be developed. However, prior to the present invention the necessary high current densities, gross currents, and long lifetimes have not been achieved in field emission cathodes.
Hopes for attaining field emission sources with currents and current densities equivalent to those of typical thermionic emitters have depended thus far on the development of multipin cathodes to yield high gross emission currents at low macroscopic fields. Spindt et al. (J. Appl. Phys., 47, December 1976, p. 5248-63) have used lithographic techniques to prepare an array of thin film molybdenum cones on a silicon wafer in a low voltage emitter configuration. The largest arrays were 5000 pins at packing densities of 6.times.10.sup.5 pins/cm.sup.2. When operated in ultra-high voltage (UHV) conditions this multipin cathode achieved current densities of 8 A/cm.sup.2 and maximum gross output current of 5 mA in direct current (dc) operation. However, the inability of Spindt et al. to prepare larger arrays and the thermal and mechanical instability of these structures prevents this approach from competing with thermionic emitters.
As an alternative to lithographic techniques, other researchers have used the rod-type structure formed upon solidification of a two-phase system that forms a eutectic in which the volume fraction of a conducting phase is much smaller than the volume fraction of the other phase. Eutectic cold cathodes of tungsten pins or rods in a matrix of uranium oxide have been the subject of the most extensive research. (See, for example, W. L. Ohlinger, Ph.D. Thesis, August 1977, Georgia Inst. of Technology.) Using a design in which the cathode and anode are separated by a vacuum gap, this eutectic cold cathode tested in UHV yielded current densities of up to 20 A/cm.sup.2 under dc conditions using small-area arrays with a density of 10.sup.7 rods/cm.sup.2. With larger cathode arrays, current densities of 1.2 A/cm.sup.2 and gross output currents of 20 mA were obtained.
Thus, although both approaches developed cold cathodes that yielded attractive current densities, neither approach was able to develop the large gross currents necessary to compete with thermionic emitters. The low gross currents are due in part to inability to fabricate large area arrays. Spindt et al. were unable to produce Mo cone arrays with more than 5000 pins, while Ohlinger was plagued by the difficulties of growing the desired large boules of W/UO.sub.2 composite. In addition to the difficulty presented by the high eutectic temperature of W/UO.sub.2, the thermal expansion coefficient mismatches and inherent brittleness of the materials resulted in extensive internal cracking in the fabricated cathode devices.
For field emission cathode devices to become practical alternatives to thermionic cathodes, they must combine stability of device structure, high current density, and high gross current output. The present invention addresses this need.